Polishing and Testing

For the polishing agent, either cerium oxide or Barnesite is preferred to rouge. After about 15 minutes of polishing, using a one-quarter diameter stroke, a test should be made to determine the benefits of the grinding.

The test, of course, should not be made immediately, but after a 15-minute wait. If the diagonal proves to be more than a wave length convex or concave (indicated in the first instance, when tested with hypothetical piece A, by straight bands, or curvature due to convexity, and in the second instance, by four or more bands concave), then the difference is best corrected by further grinding.

It is most probable, however, that any error will be within the above tolerances, and polishing can be resumed, the nature of the surface determining the method. If convex, a one-third or longer stroke can be used, in the normal positions; if concave, polishing should be done with the lap on top. The condition of the surrounding pieces does not matter, their purpose being to extend the working surface to such a size that it will not wobble or rock under the motions of grinding and polishing. On account

Fig. 55. Use of a straightedge in
testing for flatness. At a, departure
from flatness of half a wave is in-
dicated; at b, the difference amounts
to less than a quarter of a wave.
Dividers should be used for precise
measurement.

of the broken-up surface, the onion sacking should be used at all limes in pressing. When the diagonal has been completely polished, half an hour should elapse for temperature adjustment before testing.

Try to figure the diagonal as closely as possible to absolute flatness, indicated by testing two bands concave with piece A. If an optical flat is available (see Appendix B), curvature of 1/10 of a band is easily measured with straightedge and dividers, as shown above in Fig. 55. Against an optical flat, of course, an appearance like that in Fig. 52, left, indicates absolute flatness. To free the finished diagonal, place the disk in a pan of very hot water, or lay it on a board and heat it slowly in an open
oven. Any wax still adhering to the diagonal can be wiped off with a cloth dipped in turpentine.

If several diagonals have been made in the same block, as previously suggested, it may be desirable to test them one against the other; therefore, in order that the interference bands may be visible, their ground backs should be given a semipolish. This can be quickly done by charging the surface of the lap with No. 600 alundum, and scrubbing the back of each piece 011 it for a few minutes.

Or, if it is desired not to destroy the lap, grind the backs of the separate pieces on a scrap piece of glass briefly with No. 600, followed by a brief polishing on the lap.
The diagonal can now have the final bevel ground on its corners, if the 45° edging has already been done. If the edging has not been done, be careful in the procedure to avoid scratching the polished surface. This operation need not be performed, however, if the diagonal is to be made elliptical.

An Elliptical Flat